Conventionally, in a serial printer which actuates a printing means after the carriage is displaced and stopped, printing can be performed based on the letter data of one line and feed data between the letters.
For example, the printing data for "I am" consists of letter data "I", space data, space data, letter data "a", space data, and letter data "m".
As shown, printing is performed after a space data command, i.e. displacement of the carriage. However, as the carriage has inertia, damped oscillation is produced after stopping instant S of the drive means, in the form of a stepping motor, of the carriage, as shown in FIG. 1. The oscillation of the carriage is damped sufficiently by waiting a long settling time T.sub.1 after the timing S. Thus, when printing is performed after waiting this long settling time, the printing position is correct. As the time T.sub.1 is long, printing is ordinarily performed after a time T.sub.0 which is shorter than the time T.sub.1 and some deviation in the printing position is ignored. Thus, the printing speed is increased.
In such a system, problems are produced in bold print which prints the same letter by space data zero shown in FIG. 3B and in shadow print which prints the same letter by very small space data shown in FIG. 4B. In bold print, as shown in FIG. 1, first printing is performed at timing A.sub.1 after the instant S during which the drive means stops, and the second printing is performed at timing A.sub.2 which is time T.sub.1 after the timing S. Thus, the print position of the second printing is accurate as the timing A.sub.2 is sufficiently long with respect to the damped oscillation of the carriage.
In the shadow print, first printing is performed at timing B.sub.1 shown in FIG. 2, after time T.sub.0 from drive means stopping instant S, and the carriage is displaced by the necessary pitch P. Then after long settling mode time T.sub.2 which is sufficiently long for the oscillation of the carriage to be damped, second printing is performed at timing B.sub.2.
In these cases of bold print and shadow print, the second print positions are obtained accurately in the proper positions.
However, the first timing A.sub.1, B.sub.1 of the first printing is the ordinary printing timing, i.e., after time T.sub.0 from S, so that, as shown in FIGS. 1 and 2, the first printing position deviates by the dimension .DELTA.P from its proper position D. Thus, the second print position is accurately in alignment with the proper position, but, nevertheless, as shown in FIG. 3C, the printed bold print is not duplexed properly and the first and second prints deviate by .DELTA.P from each other. Also, in the shadow print, as shown in FIG. 4C, the first and second prints vary by a distance P.sub.1 which is vider than the intended pitch P or by a distance narrower than the intended pitch P. Therefore, they have had this fault to lead to the printing of unbecoming letters.